The present invention relates to a method for the detection of local displacements and rotations and to a device for the local frequency doubling of moving incremental scales to detect local displacements and rotations.
Devices for the detection of rotational speeds in motor vehicles, as represented in FIG. 1, are principally known in the art. They comprise an encoder and a sensor magnetically scanning the encoder via an air slot. The encoder is a machine element, which is mechanically coupled to the rotating ring of a wheel bearing and carries an incremental angle scale. Said angle scale is designed as an integral sequence of magnetically alternating, differently effective areas forming a circular encoder track. It is conventional practice to use as encoders toothed wheels, ferromagnetic perforated discs or permanent-magnetized structures, e.g. magnetized wheel bearing seals. The sensor reacts to the periodic changes tooth/gap or hole/web or north/south pole with a periodic electric signal reproducing the incremental angular spacing as a temporal voltage or current variation. Induction coils, magnetoresistive bridges and Hall elements are used as sensorially active components, being partly operated in combination with additional electronic circuits. It is usual to designate sensors as ‘active sensors’ when they require a current supply for operation and as ‘passive’ sensors when they do not need an additional current supply for operation, exactly as induction coils.
EP-A-0 922 230 (P 8775) describes an arrangement for the detection of the rotational behavior of a rotating encoder, with a sensor module comprising the following functional groups: a sensor element on the basis of the magnetoresistive effect, a controllable current source supplying a load-independent current representative of the rotational behavior, and a modulator controlling the current source in response to signals of the sensor element. The sensor module is magnetically coupled to the encoder. The output signal is a signal representative of the rotational behavior with superposed status signals and/or additional signals. The status signals contain among others the information about the direction of rotation.
WO 99 49322 (P 9352) discloses an interface where the information about the direction of rotation and its validity is contained as two-bit information within an 8-bit word being sent after each rotational speed pulse. Further, active sensor elements on the basis of the Hall effect are known (TLE 4942, Infineon Technologies, Munich) wherein, apart from the number of rotations, also the information about the direction of rotation is transmitted in a coded form. As this occurs, the signal alternates between two current levels. The temporal distance of the leading edges signals the wheel speed, while the direction of rotation is coded based on different pulse durations.
Proposals have already been made to bisect the incremental bearing discrimination to reach higher air gaps and to compensate these thereafter by doubling mechanisms with the use of sensors locally shifted in relation to each other. Thus, DE 199 06 937 recommends using two Giant Magnetoresistive Effect sensors whose local positioning in relation to each other brings about a phase shift of roughly 90°. The signals of the two sensors are amplified, led through threshold value switches and exclusively OR-operated. Another objective is to determine the direction of rotation by means of flip-flop circuits. Further proposals are directed to arranging the sensors on one joint substrate in order to be able to maintain the distance between both sensors as precisely as possible.
The application of this prior art is obstructed in several ways in practical operations. Thus, it is necessary to combine identical sensors with encoders of different modules (module=reading diameter/encoder number of cycles) for the case of application of the detection of the rotational speed in an automobile. According to experience, the module range is between 1.2 mm and 2.5 mm, that means, a ratio of 2.5/1.2=roughly 2 must be covered. In order to always preserve a phase shift of roughly 90°, it would be necessary according to the state of the art to keep on stock a large number of different sensors adapted to different modules. This necessity counteracts the goal of an economical manufacture and quality of large piece numbers of an equal product. When module adaptation is omitted, another shortcoming occurs because, with increasing phase deviations from the nominal value of 90°, each of the exclusively OR-operated sensorial channels contributes an individually fluctuating pulse-duty factor to the total signal, with the result of inadmissibly increasing the jitter for the operation of modern brake controllers. To use a signal of this type, the effort required in the ECU is significantly higher compared to conventional sensors where the signal frequency follows the encoder frequency. Usually, the evaluation is made from leading to leading edge and from trailing to trailing edge in order to determine the wheel rotational speed. Jitter errors being caused by an unsymmetrical pulse-duty factor are hereby prevented. When two partial signals are mixed according to the proposal made in the above-mentioned DE 199 06 937, the error arising will be doubled because the ECU is now additionally required to make a distinction between two groups of edges, namely leading and trailing edges of the one partial channel and leading and trailing edges of the other partial channel. However, this also implies that a signal frequency, which undergoes evaluation in the described fashion, is not actually doubled. Instead, only the single encoder frequency is determined twice without additional efficiency. Thus, the prior art indicated (in DE 199 06 937) demands an exact phase shift of 90°, i.e. an absolute module adaptation, for its practical utilization, which is disadvantageous for the mentioned reasons.
Thus, an object of the present invention is to produce a signal with a double temporal number of cycles with one sensor per encoder angular period, e.g. north/south pole pair, tooth/gap. More specifically, the object of the invention is to reach a doubling of the local frequency of incremental scales being read by sensors by means of a field coupling effect. The invention is implemented in particular in the detection of linear travel shifts and/or angle shifts or the detection of associated motional speeds or rotational speeds in the automotive industry.